An organic semiconductor is expected to be applied to a photoelectric conversion element such as a solar cell, a light emitting element, and a photosensor. Especially, using a high molecular compound as an organic semiconductor material enables application of a low-cost coating method in fabrication of an active layer. In view of an energy demand and an emission reduction of CO2, a solar cell is expected as one of clean energies with low environmental burdens and its demand is rapidly increasing. A silicon-based solar cell is mainstream at present, but efficiency thereof is about 15%, and it is difficult to curtail a cost. A CdTe solar cell is also known as a solar cell which can be fabricated at a low cost. However, since Cd being a toxic element is used, there is a possibility that an environmental problem occurs. Under the circumstances, practical use of an organic thin film solar cell and an organic/inorganic hybrid solar cell is expected as a next-generation solar cell which is low in cost, high in energy conversion efficiency, and nontoxic.
In order to put an organic thin film solar cell to practical use, improvement of power generation efficiency is intensely demanded, and for this purpose, organic thin film solar cells using organic semiconductors made of a variety of high modular compounds are studied. For example, as a polymer exhibiting conversion efficiency as high as 7% or more when applied to an organic thin film solar cell, there is known poly(4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4-(2-ethylhexyloxycarbonyl)-5-fluoro-thieno[3,4-b]thiophene-2,6-diyl) (abbreviated form: PTB-7). Further, there is also known a polymer (abbreviated form: PBDTTT-CF) made by substituting a carbonyl group (n-heptylcarbonyl group) which is strong in electron withdrawing property for an ester substituent at 4-position of a thieno[3,4-b]thiophene ring of PTB-7, in order to improve an open circuit voltage (Voc) of an organic thin film solar cell.
Synthetic pathways of monomers of the aforementioned polymers (PTB-7 and PBDTTT-CF) are multi-stepped and quite complicated. In order to decrease synthetic stages, there is developed poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4-(2-heptylcarbonyl)-thieno[3,4-b]thiophene-2,6-diyl} (abbreviated form: PDBTTT-C) in which hydrogen is substituted for fluorine of 5-position of thieno[3,4-b]thiophene of the PBDTTT-CF, but with this polymer, conversion efficiency is slightly decreased. With regard to the PDBTTT-C, it is known that the conversion efficiency is improved by replacing a 2-ethylhexyloxy group of substituents of 4-position and 8-position of a benzo[1,2-b:4,5-b′]dithiophene ring with a 5-(2-ethylhexyl) thienyl group.
As described above, conversion efficiency of a solar cell is largely affected not only by a skeletal structure of an organic semiconductor made of an organic high molecular compound but also by a side chain of the organic high molecular compound. Therefore, in order to improve performance of an organic thin film solar cell using the organic semiconductor, it is required to improve the skeletal structure and the side chain of the organic high molecular compound. Further, improvement of a life property in addition to improvement of an open circuit voltage is required of the organic thin film solar cell. In order to improve the life of the organic thin film solar cell, active substances (a donor and an acceptor) excellent in light stability and heat stability are required.
Further, researches have recently been made on an organic/inorganic hybrid solar cell whose energy conversion efficiency is improved by using an organic/inorganic hybrid perovskite compound or an inorganic perovskite compound for a photoelectric conversion layer. In the organic/inorganic hybrid solar cell, polyarylamine or 2,2′,7,7′-tetrakis(N,N-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) is used as a hole transport layer. Further, in order to enhance conversion efficiency, a dopant such as t-butylpyridine (TBP) or bis(trifluoromethanesulfonyl)imidelithium (Li-TFSI) is used. However, since TBP is liquid and Li-TFSI is a hygroscopic substance, there occurs performance deterioration caused by diffusion or dissipation of TBP to the photoelectric conversion layer due to a temperature increase, by absorption of water molecules due to deliquescence of Li-TFSI, and so on. This is a cause to shorten the life of the organic/inorganic hybrid solar cell. It has been also proposed to use P3HT being a p-type material as the hole transport layer, but sufficient power generation efficiency cannot be obtained in this case.